The congressman will make a formal announcement when he’s back in New Mexico during Congress’s Thanksgiving recess, Nagle said.
Udall’s decision means a wide-open congressional races in all three New Mexico districts in 2008.

The state’s other two members of Congress, Republicans Heather Wilson and Steve Pearce, are giving up their seats to run for the GOP nomination for Domenici’s seat.

Domenici plans to retire in January 2009 because of an incurable brain disease.

Udall, who was elected to northern New Mexico’s 3rd District seat in 1998, is a former two-term state attorney general and the son of former Interior Secretary Stewart Udall.

He faces Albuquerque Mayor Martin Chavez and other, lesser-known Democrats in the party’s June primary election.

Udall initially had said he would stay in the House, where he is a member of the Appropriations Committee.

But he reconsidered after a “groundswell” of encouragement from New Mexicans, Nagle said.

“He thinks it’s too important not to run for the Senate,” Nagle said. “A lot of the good things we’ve been able to do (in the House) don’t go anywhere in the Senate.”

Nagle said there has been criticism that Udall is “too liberal” for the seat but he disputed that, saying “the rhetoric doesn’t match the reality of what the data show.”

He said internal campaign polling of likely voters conducted for Udall at the end of October showed the congressman with a considerable lead over either Wilson or Pearce, including among self-described moderates.

The polling also showed Udall with a strong lead among likely Democratic primary voters over Chavez and Santa Fe developer Don Wiviott, Nagle said.

Chavez said Saturday that he isn’t backing down from the race because of Udall’s decision to run.

“I’m running for the reasons I stated when I announced, and if anybody wants to get in, this is a democracy,” Chavez said, adding that he believes he “absolutely” can beat Udall.

“At the end of this, it’s really about the failure of Washington to change, and I’m the only candidate with a record of strong leadership” in creating change, he said.

Other Democrats who have said they will run for the nomination include Jim Hannan, finance director of the Santa Fe Community Housing Trust, and Leland Lehrman, who edits an alternative newspaper in Santa Fe.

Udall has about $800,000 in a congressional campaign fund that he can use for the Senate race, according to Nagle.

Udall is a cousin of Mark Udall, the Democratic congressman from Colorado who is also running for the Senate, for the seat that will open up because of the retirement Republican Wayne Allard.

Born in Tucson, Udall grew up in Arizona and in the Washington, D.C., area. His father was an Arizona congressman for six years and then headed the Interior Department for eight years under Presidents Kennedy and Johnson. His uncle, Morris, was in the U.S. House for three decades.

Udall got his bachelor’s degree from Prescott College in Arizona and went to law school at Cambridge University in England and at the University of New Mexico.

He ran unsuccessfully for Congress as the Democratic nominee in the Albuquerque-area 1st District in 1988, and was elected attorney general in 1990 and 1994.